In the production printing (PP) market, there is a strong demand for stable imaging performance resulting in consistently high quality of the output images. However, particularly with image forming apparatuses, it is difficult to prevent a change in halftone characteristics of colors based on a change over time in printer engine characteristics, and accordingly it is important to correct the halftone characteristics with the proper timing. To address this issue of correcting output images, a halftone correction device is widely used, which prints a special pattern chart, measures the colors of the chart using a scanner installed in an image forming apparatus, and refers to a correction table. The existing halftone correction device, however, prompts a user to perform the printing and scanning processes. Thus, the measurement and correction of the colors may fail to take place in a timely manner.
In view of the above, a halftone correction method is disclosed in JP-2009-211641-A that draws patches for correction on an electrophotographic intermediate transfer belt, measures the colors of the patches using image density sensors originally provided for controlling the solid density, and corrects the halftone. The method performs the halftone correction in a timely manner, specifically after solid density control is executed based on color and tone correction process. The method, however, forms a relatively large number of patch patterns with each correction. A dithering process serving as a halftone process includes various techniques, such as dot screen, line screen for 2-bit data, line screen for 4-bit data, and hybrid thereof. Particularly when a plurality of halftone processes (i.e., dithering processes) are performed, the number of patch patterns is multiplied by the number of halftone correction processes. The increase in the number of patch patterns increases the halftone correction time and the amount of toner consumed in the halftone correction processes, and thus imposes a burden on a user.
In view of the above, JP-2008-244644-A discloses a technique in which, when a plurality of screen dot processes are performed, the density of an image formed by a screen dot pattern generated with a specific number of lines is measured, and a screen dot pattern having another number of lines and not subjected to the measurement is estimated using a predetermined function model, to thereby generate a correction table. This method, however, is applicable only to estimation between screen dot patterns, and cannot comprehensively handle a variety of halftone processing patterns including screen line patterns.